I got a postcard from my sister who is traveling in England. It shows four horses made of white stones surrounded by grass. I have seen pictures of this ancient(?)artwork, and I know there is a story behind it, but can't remember the details. Can someone fill me in?

I can just make out the words Westbury, Pewsey, Uffington and Cherhill, on the back of the card. I assume those are place names, but I haven't gotten to the map yet.
Howard